Aircraft use various control surfaces that, through interaction with the airflow around the aircraft, produce forces and moments about the aircraft center of gravity to provide flight path control. Primary flight control surfaces can include ailerons for roll control, elevators for pitch control, and rudders for yaw control. Conventional lift and drag devices can include leading edge devices, trailing edge flaps, spoilers, and speed brakes.
FIG. 1 is a partially schematic top view of a conventional aircraft wing 1, having a low speed aileron 2, a high speed aileron 3, an outboard flap 4, an inboard flap 5, flight spoilers 6, ground spoilers 7, and leading edge devices 8 configured in accordance with the prior art. Each control surface is designed to provide specific functionality. For example, the high speed aileron 3 is designed to provide roll control when the aircraft is operating at high speed. The flight spoilers 6 are designed to augment roll control throughout the flight envelope. During low speed operation, the low speed aileron 2 is designed to augment the roll control provided by the high speed aileron 3 and the flight spoilers 6. The inboard flap 5 and the outboard flap 4 are designed to provide various high lift configurations used for takeoff and landing. The flight spoilers 6, in addition to augmenting roll control, are designed to provide a high drag configuration in flight. On the ground (e.g., during landing rollout or an aborted takeoff) the flight spoilers 6 and the ground spoilers 7 are designed to provide drag to aid in slowing the aircraft. Additionally, on the ground, the flight spoilers 6 and the ground spoilers 7 are designed to decrease the lift produced by the wing, placing more weight on the wheels, and thereby increasing the effectiveness of the wheel brakes.
The typical design process, which yields the design depicted in FIG. 1, includes determining the operating requirements of the aircraft, the location and size of the control surfaces, the flight control laws that will be used (if any), and then determining the actuator size for each control surface. Determining the operating requirements includes defining vehicle characteristics, the environment in which the vehicle will operate, and various operating states that the vehicle may experience. For example, operating requirements for an aircraft can include, among other things, the aircraft's characteristics such as size and weight, takeoff distance, landing distance, normal load factor capabilities, range, stall characteristics, descent capabilities, climb capabilities, and a range of operating airspeeds. Environmental factors can include, among other things, an operating range of air density, an operating range of air temperature, and environmental discontinuities such as gusts and wind shears. Operating states can include, among other things, a carriage of external stores, the capability to operate with a power plant failure, and/or the capability to operate with a flight control surface that is jammed such that it will not move from a fixed position.
As noted above, after the operating requirements are defined, the location and estimated size of the various control surfaces needed to satisfy the operating requirements are chosen. Typically, conventional control surface locations are chosen on the wing and tail sections of the aircraft. The size of the control surfaces results from the amount of surface area needed to generate the forces and moments to satisfy the operating requirements. Control surfaces may b actuated by pure mechanical means through hydraulic actuators or by fly-by-wire or fly-by-light systems using a computer-based set of control laws that command the appropriate deflection of the control surface needed to satisfy the operating requirements and achieve the commanded flight path or aircraft response.
If a set of control laws is used, it may be an augmentation system, which provides command signals to the control surfaces in addition to pure mechanical inputs, or a full authority control system where there are no direct mechanical links to the control surfaces. In either case, the set of control laws is typically implemented using a combination of sensors and look-up tables to determine the appropriate control surface movement to satisfy the operating requirements and achieve the desired flight path or aircraft response. Typically, the set of control laws is designed with the operating requirements in mind and tailored during the design process to the size and location of the control surfaces used.
After the control surfaces have been sized and located, and the set of control laws defined (if applicable), the required actuator characteristics needed to satisfy the operating requirements are determined for each control surface. These characteristics can include, but are not limited to, actuator rate capabilities, actuator force capabilities (instantaneous and prolonged), damping characteristics, blow-down characteristics, and response to a loss of hydraulic pressure if a hydraulic actuator is used. The required actuation capability is determined for each control surface based on the operating requirements, control surface size, and control surface functionality.
For example, an aileron providing roll control is required to move rapidly so that an operator can maneuver the aircraft during various phases of flight, including landing or in response to an upset caused by a gust of wind. Conversely, on a transport aircraft, the flaps generally do not need to move rapidly since they are typically used to configure the aircraft for longer term tasks, including takeoff, landing, or cruise. These configurations generally maintain the flaps in a fixed position for at least several minutes, and the flaps often take several seconds to transition to the various positions. Accordingly, different control surfaces will require actuators with different actuation rate capabilities.
Similarly, different control surfaces will have different actuation force requirements. For example, the actuator operating a spoiler panel or an aileron may not be required to generate as much force as an actuator operating a larger surface such as a flap. Other actuation requirements vary from control surface to control surface in a similar manner. This variation in requirements results in the need to use actuators with different capabilities throughout the control system in order to satisfy the operating requirements given the size, location, and function of each control surface.
One shortcoming of the prior art design process, and resulting design, is that the aircraft is built with numerous specialized control surfaces, positioned by numerous actuator types having diverse capabilities. This increases the design and manufacturing costs of the aircraft because numerous types of actuators with varying characteristics must be designed, manufactured, and inventoried for use in production. These different actuator types often have diverse installation instructions and functional test requirements, further increasing the cost of production. Additionally, the prior art design process also increases the operating cost of the aircraft because operators are required to maintain spares for each of the numerous actuator types installed in the aircraft. This increases the cost of maintaining an inventory of spare parts because it increases the physical number of spares that must be kept on hand. It also makes for a complex supply chain because the correct actuator type must be identified, located in storage, and transported to the maintenance point without error. A further shortcoming of the prior art design process, and resulting design, is that control law sets are tailored to specific control sizes and locations. Accordingly, it can be difficult to adapt the control law set of one vehicle to other vehicle designs, and/or other operating requirements.